1. Field of the Invention
The present invention relates to a piezoelectric film element used as a micro-actuator for, for example, an ink jet recording head or a micro-pump, and a method of manufacturing such a piezoelectric film element, and an ink jet recording head using the piezoelectric film element as a drive source for discharging ink. More particularly, this invention relates to a piezoelectric film element concerning which a piezoelectric film with excellent piezoelectric properties can be selectively formed by a hydrothermal synthesis, and a method of manufacturing such a piezoelectric film element, and an ink jet recording head using the piezoelectric film element as a drive source.
2. Description of the Related Art
As an element for converting electric energies into mechanical energies, a piezoelectric film element is known which is constructed in a manner such that a piezoelectric film is held between a lower electrode as a common electrode and an upper electrode as an individual electrode.
This piezoelectric film element is used, for example, as an actuator for, for example, an ink jet recording head. When the piezoelectric film element is used, for example, as an actuator for an ink jet recording head, a piezoelectric film is normally required to have a film thickness ranging approximately from 1 .mu.m to 10 .mu.m.
As the piezoelectric film, for example, a film made of lead zirconate titanate (hereinafter referred to as a "PZT" film) is used in general. This PZT film is manufactured by a physical vapor deposition (PVD) method such as sputtering, a chemical vapor deposition (CVD) method, a spin coating method such as a sol-gel method, or a hydrothermal synthesis method.
Among these film-forming methods, the use of the hydrothermal synthesis has often been examined recently because it is possible to make a reaction proceed in a low temperature environment at 200.degree. C. or lower and to inhibit the generation of cracks.
As stated in "Preparation of PZT Crystal Film by Hydrothermal Synthesis and Electrical Properties Thereof" from lecture proceedings (published on Oct. 2, 1995) of the 15.sup.th Electronic Material Studies and Symposium held by the Ceramic Society of Japan, this hydrothermal synthesis comprises: a seed crystal forming process for causing PZT seed crystal to be deposited on the surface of a titanium metal substrate; and a crystal growing process for causing PZT crystal to be deposited and to grow over the PZT seed crystal.
More specifically, the official gazette of the Japanese Laid-Open (Kokai) Publication No. HEI 8-306980 discloses a method of manufacturing a piezoelectric element unit by utilizing this thermal synthesis. It is described that as a first step thereof, a titanium film is formed by a mask film forming method of forming a film only at a specified position of a substrate which does not contain titanium, or a titanium film is formed over the entire surface of the substrate, then the titanium film is caused to remain only at portions where PZT is formed by a photo-etching method. Thereby, the PZT is formed over the titanium film.
It is described that as a second step, platinum as a protective film is formed over the substrate made of titanium at portions where the PZT film is not formed, or the protective film formed over the entire surface of the substrate undergoes etching in accordance with the pattern for forming the PZT.
According to the above-described hydrothermal synthesis, however, it is inevitable that the PZT seed crystal will be formed also over the protective film. When forming a minute pattern as in the case of an ink jet recording head, since a PZT seed density on the titanium film is higher than a PZT seed density on the protective film, the PZT seed crystal over the titanium film shows a higher growth rate and, therefore, the PZT film obtained as a result of growth of the PZT seed crystal on the titanium film connects with the PZT film obtained as a result of growth of the PZT seed crystal on the protective film, thereby causing a problem in that the protective film and the PZT film formed thereon cannot be removed. Consequently, there is a problem in that the PZT film cannot be formed selectively in a specified area.
When the substrate is made of electroformed nickel, if gold or the like is used as a material for forming the protective film, a local battery is formed between gold and nickel upon wet etching of the protective film and then nickel suffers electrolytic corrosion, thereby causing a problem in that the dimensional precision lowers.
Particularly the piezoelectric unit element described in the official gazette of the Japanese Laid-Open (Kokai) Publication No. HEI 8-306980 has the following problems. As a first problem, when the protective film has undergone patterning by means of etching and when a PZT element is formed by utilizing a hydrothermal reaction, an alkali solution used in the hydrothermal reaction or an etching reagent penetrates through openings of the protective film into the inside of the protective film, thereby causing deterioration of adhesion between the protective film and the substrate and giving rise to a possibility that the protective film may come off from the substrate.
As a second problem of this example of the prior art, when the thickness of a platinum film as the protective film is thin, the platinum film fails to fulfill its function as the protective film and the PZT seed crystal will be deposited on platinum, so that such a minute pattern will not be formed as the ink jet recording head would demonstrate.